Conventionally, a mobile communication system capable of accommodating a radio access network of 2G/3G scheme (UTRAN: Universal Terrestrial Radio Access Network or GERAN: GSM EDGE Radio Access Network), and a radio access network of LTE (Long Term Evolution) scheme (EUTRAN: Evolved Universal Terrestrial Radio Access Network) has been known.
An “SRVCC (Single Radio Voice Call Continuity) scheme” is specified in 3GPP TS23.216. The SRVCC (Single Radio Voice Call Continuity) scheme is for switching a voice communication path between a UE (User Equipment) #1 and a UE#2 from a path via E-UTRAN or UTRAN (a path for VoIP (Voice over IP) communications) to a path via UTRAN/GERAN (a path for circuit-switched (CS: Circuit Switch) communications).
In addition, Alt.4 or Alt.11 of 3GPP TR23.856 v 0.4.1 and the like specifies an SRVCC scheme (hereinafter referred to as an improved SRVCC scheme) for switching a voice communication path between a UE#1 and a UE#2 from a path via E-UTRAN or UTRAN to a path via UTRAN/GERAN, with an MGW (Media Gateway) #1 in a visitor network of the UE#1 as an anchor point.
Such improved SRVCC scheme enables termination of a signal necessary for switching the voice communication path between the UE#1 and the UE#2 in the visitor network of the UE#1, and can reduce communication outage time due to back and force transmission of such signal between the visitor network of the UE#1 and a home network of the UE#1, compared with the conventional SRVCC scheme.
Here, FIG. 12 shows an originating process from the UE#1 to the UE#2 in a mobile communication system which can use the above-mentioned SRVCC scheme and the improved SRVCC scheme. FIG. 13 shows a registration process with an IMS of the UE#1 in a mobile communication system which can use the above-mentioned SRVCC scheme and the improved SRVCC scheme.